Larger chainring and sprocket.

[mickeg]

Is there any obvious reason why no one seems to be using the now readily available 50 x 21 tooth combination with the Rohloff? With 26" x 2 tyres this would give a very useful touring ratio of 90.8 to 17.3, along with longer wearing cogs and chain. Am I missing anything?

I think the consensus is that there is an obvious reason.  We all started out with a different smaller sprocket and chainring.  And we all plan to use what we already own until a sprocket or chainring or both wear out.  Once they wear out, maybe make a change at that time. 

But those that use the chainglider are more likely to not make any changes since they have something that they are content with.

The advantage of the bigger sprocket and chainring for longer chain life and possibly longer sprocket and chainring life is not in question for those of us that do not use a chainglider.

I plan to stay with even numbered sprocket and chainrings, but it will be years before I run out of the old threaded sprockets.

[martinf]

Isn't there an efficiency advantage with bigger rings?

Yes. But I don't know if going from 38x16 to 50x21 makes much difference to efficiency, my experience would suggest not.

Very small sprockets are significantly less efficient, here I am thinking 11T as used on a Moulton versus the 14T top gear sprocket on my derailleur lightweight. Found a summary here that gives an idea of the efficiency gain going from 13T to 24T:

https://www.cyclingpowerlab.com/DrivetrainEfficiency.aspx

For me, the most noticeable difference in going for larger rings/sprockets is durability.

Past experience with derailleur bike half-step triples suggests that 44/50 lasts significantly longer than 40/36, both these were with small inner rings (26 to 30), wear on the latter is fast if used often, which wasn't the case as mountain touring only accounted for a fraction of my riding.

On utility bikes with hub gears, 44x22 was slightly more durable than 38x19 before I discovered the Chainglider.

I suspect that the main effect as far as durability is concerned is faster chain wear on a small sprocket leading to faster wear on the chainring(s).